1. Field of the invention
The present invention relates to a surface shape measuring apparatus and a surface shape measuring method for measuring the shape of a curved surface of a work.
2. Description of the Prior Art
Recently, the automobile industry is making efforts to reduce a body weight in improving fuel efficiency and reducing emissions. Under these circumstances, there is a tendency to make a steel plate thinner, which, however, raises a problem that a strength of the steel plate becomes insufficient. A countermeasure against an insufficient strength of the steel plate is taken by making a thin steel plate from a high-strength material, such as a high-tensile steel plate, or providing a reinforcing bar. Using a high-tensile steel plate, however, raises a problem that the manufacturing costs are increased in comparison with typical steel plates. Also, because a reinforcing bar is made of metal, there is a problem-that providing the reinforcing bar has an adverse effect on a weight reduction.
With this being the situation, an increasingly adopted method for suppressing an increase in weight while compensating for a reduction in rigidity is to reinforce a thin steel plate by sticking a sheet of reinforcing material made of resins.
This steel plate reinforcing method with the use of a sheet of reinforcing material can indeed reinforce a steel plate while suppressing an increase of the steel plate in weight; however, as the steel plate is made much thinner to meet the need in recent years, a distortion called “sink” may occur on the surface of the steel plate. This distortion is an extremely minute deformation of the steel plate, and is hard to identify by merely taking a glance. In addition, this distortion is normally checked by visual inspection, and it is therefore difficult to quantify this distortion.
To solve these problems, an attempt is being made to identify a distortion on the surface of the steel plate using a three-dimensional shape measuring apparatus disclosed, for example, in PCT Unexamined Patent Publication No. 2003-533685.
The three-dimensional shape measuring apparatus disclosed in PCT Unexamined Patent Publication 2003-533685 supra measures an object to be measured with a photopolarimetric sensor. The photopolarimetric sensor, however, has a measurable distance range with respect to an object to be measured, and measuring accuracy is low when the measurable distance range is wide whereas measuring accuracy is high when the measurable distance range is narrow.
When the surface of a steel plate is a curved surface, a distance between the photopolarimetric sensor and the curved surface on the surface of the steel plate in a confronting direction is not constant and varies along a curved direction, which raises the need to use a photopolarimetric sensor with a wide measurable distance range.
However, because measuring accuracy of a photopolarimetric sensor with a wide measurable distance range is low, it is difficult to detect a distortion precisely.
In particular, when a coating is applied on the surface of the steel plate, gloss of the coating makes it quite difficult to detect a minute distortion precisely.